1. Field of the Invention
This invention relates in general to starting of gas discharge lamps and more particularly, to rapid restarting of high pressure sodium lamps.
2. Background
Some high intensity discharge (HID) lamps utilizing metal vapors as the discharge medium require a starting voltage much higher than the operating voltage and this is particularly so with high pressure sodium (HPS) vapor lamps. For such lamps, starting aids have become available which are combined with otherwise conventional ballasts to generate a series of high frequency pulses which initiate the breakdown in the lamp.
The high pressure sodium lamp uses xenon as a starting gas because it will start conducting more easily than sodium vapor. When the lamp is cool, a standard starting pulse, about 2.5 kilo volts (KV), ionizes the xenon which causes the lamp to conduct. When the lamp is hot, after continuous operation, the sodium has vaporized which increases pressure in the lamp. A normal starting pulse will not restart the hot lamp because the vaporized sodium gas is not ionized as easily as xenon. As the lamp cools, the vapor pressure of sodium decreases and relative concentration of xenon increases. A lamp with a standard starter must cool for about one minute before it will restart. Pulses from the starter while the lamp cools will have no effect.
HPS lamps can be restarted while hot but it requires a higher voltage pulse to do so. The high pressure sodium lamp requires a pulse of about 7 KV to restrike a hot lamp. The definition of what constitutes a rapid restrike depends on the manufacturer but typically a hot lamp may take from 0 to 40 second to restrike or restart depending on the pulse voltage, waveshape, and the lamp characteristics.
There are several rapid restrike devices currently available. One such device described in U.S. Pat. No. 4,527,098--Owens, Discrete Starter for HID Lamp, uses a sidac as a voltage trigger which discharges a capacitor into the windings of a transformer which then produces a high voltage pulse. The transformer isolates the pulse from the ballast. A power resistor in this design is an undesirable additional heat source.
Another rapid restrike device is described in U.S. Pat. No. RE 31,486--Helmuth, Rapid Starting of Gas Discharge Lamps. This restrike device is comprised of a solid state circuit triggered when the ballast output is 120 volts RMS with the lamp off. An output pulse is formed by a capacitor discharging into the primary of a transformer which has a high turns ratio so that high voltage pulses are formed. There are two transformer secondary windings, one on each lamp lead and both the lamp socket and the lamp shell are pulsed. An electron gun is also used as a source of ionization. This configuration generates such a great amount of heat that it requires a thermal cutout. The thermal cutout will sometimes interrupt restrike pulses delaying restarting of the lamp.
An additional problem with some prior art rapid restrike devices is that the capacitors used for the restart circuit remain in the circuit during operation of the lamp. This creates a problem in that it causes spikes on the leading edge of the square wave generated by the ballast which can lead to shorter lamp life.
It is, therefore, an object of the present invention to provide an improved rapid restrike starter which removes the starting capacitance during operation of the lamp.